Tanks employed for the transportation of bulk chemicals, usually in liquid form, are required to meet various safety standards established by federal, state and local governments to protect the population and/or environment from any inherent hazards caused by leakage of the chemicals from the tanks. Frequently, the chemicals are required to be transported at a pressure above or below atmospheric. Heretofore, metallic tanks have been employed to meet the requirements of pressure capability as well as handling, such as stacking, loading, potential mechanical abuse, availability of approved materials and standard approved fittings, and economy. However, material which reacts chemically with metals naturally presents problems when transported in metal tanks, resulting in corroded tanks which might develop into dangerous leaks, and sometimes ruinous alteration of the chemical being transported.
Transportation of these chemicals in polyolefin tanks may eliminate the problem of chemical reaction with the tank but the inherent lack of mechanical strength of a polyolefin tank makes it undesirable from an economic and safety viewpoint.
In order to obtain the advantages of both metallic and plastic tanks, it has been proposed to line the inside of a metallic tank with a plastic or other material which is compatible with the chemical to be transported. These lined tanks usually consist of a lining of plastic materials either adhered to the inside surface of the metallic tank, or inserted into the metallic tank in the form of a thin, flexible bladder.
In tanks having the plastic lining adhered to the interior surface thereof, the plastic is applied by spraying or securing a laminate plastic material to the tank wall by a suitable bonding agent resulting in a lining having seams, joints, and a nonuniform thickness. Unless a highly skilled workman is employed to apply the plastic liner to the tank, voids and pinholes occur in the liner resulting in early failure of the tanks and accompanying safety hazards and loss of economy. Furthermore, there is a substantial difference in the thermal coefficients of expansion between the plastic liner and metallic tank often causing rupture of the bond between the liner and tank, resulting in tearing or splitting of the lining, which can also occur by a physical blow to the outside of the metallic tank.
Tanks having the bladder-type linings have not been satisfactory in many applications due to the relative ease of damaging the thin, flexible bladder during filling, handling and cleaning.
After considerable research and experimentation, the polyolefin lined tank of the present invention has been devised to overcome the disadvantages experienced with the above-mentioned lined tanks, and comprises, essentially, a rigid polyolefin tank mounted within a metallic tank wherein there is no adherence or bonding between the walls of the inner polyolefin tank and the outer metallic tank, whereby damage to the tank through thermal expansion and contraction is prevented since the inner polyolefin tank and outer metallic tank are free to expand and contract independently of each other. Furthermore, by having no adherence between the walls of the inner polyolefin tank and the outer metallic tank, damage to the inner tank by a physical blow to the outer metallic tank is minimized.
The polyolefin tank is fabricated within the metallic tank by a state-of-the-art rotational molding technique wherein a high temperature paint or other suitable release agent is first applied to the inner walls of the metallic tank, and then polyolefin powder is placed in the metallic tank which has been pre-heated. The polyolefin melts during rotation of the continuously heated tank to thereby coat the inner walls of the tank. The tank is then cooled, resulting in shrinkage of the polyolefin away from the metallic walls. During the cooling cycle, the interior of the vessel is pressurized to control the shrinkage, resulting in minimum clearance between the outer and inner tanks, and stress relief of the inner polyolefin tank. Thus, a polyolefin tank within a metallic tank is provided wherein the polyolefin tank is continuously molded, resulting in a seamless vessel of uniform thickness, free of voids or cracks. By using state-of-the art rotational molding techniques, the thickness of the inner polyolefin tank can be controlled to provide a heavy walled, rigid vessel, possessing mechanical strength and durability superior to thin flexible linings.
Fixtures are attached to the flanges of the fittings of the metallic tank to allow the polyolefin to flow through the metallic tank fittings to thereby coat the outer surfaces of the metallic tank flanges. Thus, the complete neck and flange are formed on the inner polyolefin tank during the rotational molding process, thereby precluding the necessity for weldments, bonds, or seams to attach or finish the flange. This permits the inner tank's flanges to be sealed between the outer tank's flanges and polyolefin lined closures, assuring the integrity of product containment entirely within the inner polyolefin tank.